The present invention relates to a testing system for detecting and measuring the transmission rate of certain volatile organic chemicals through a barrier material, such as a thin plastic film.
The measurement of the rate of permeation of certain selected gases through film material is known in the art. For example, measurement of the permeation rate of oxygen through plastic film materials has been accomplished by various products manufactured by the assignee of the present invention. Oxygen permeation measuring systems typically utilize an oxygen sensor of the type described in U.S. Pat. No. 3,223,597, Hersch, which are uniquely capable of sensing minute quantities of oxygen in a carrier gas, and developing an electrical signal proportional to the quantity of oxygen sensed. Detectors of this type are generally known as coulometric sensors; while extremely useful for measuring quantities of gases such as oxygen, they are not usable for detecting quantities of organic materials conveyed by gaseous transmission.
The measurement of organic chemicals conveyed in gases can be accomplished by the use of a flame ionization detector, particularly when the substance to be measured produces charged ions when burned in a hydrogen-air flame. The combustion forms ions and free electrons which may be detected by electrodes placed in the flame ionization detector. A high voltage potential is imposed across the electrodes, which are spaced apart to form a gap, and the free ions and electrons lower the resistance across the gap and causes a current to flow. This current flow is measured as a voltage drop across an external resistor; the voltage drop is amplified and interpreted by an electrometer to provide a voltage representative of the characteristics of the gas. For example, when CH.sub.2 chemical compositions are introduced into the flame a complex process takes place in which positively-charged carbon ions and electrons are formed, and the measured current is significantly increased in response to the additional ions and electrons. In an organic compound the electrical response is proportional to the number of oxidizable carbon atoms, and therefore can provide a measure of the type of organic compound. For example, butane has twice as many carbon atoms as an equivalent volume of ethane.
The flame ionization detector is insensitive to chemical compositions having fully oxidized carbons, such as carbonyl or carboxyl groups and ethers. The flame ionization detector does not respond to inorganic compounds apart from those easily ionized in a hydrogen/air flame at a relatively high temperature. It is also insensitive to water and permanent gases such as CO, CO.sub.2, CS.sub.2, SO.sub.2, H.sub.2 S, NH.sub.3, N.sub.2 O, NO, NO.sub.2, SiF.sub.4, and SiCl.sub.4. Therefore, the flame ionization detector is particularly useful when it is required to measure small traces of organic materials borne by such permanent gases.
The flame ionization detector is a mass flow detector, and its performance varies with the flow rate of a carrier gas, the voltage applied across the detector electrodes, and the temperature of the flame. The temperature of the flame is a function of the hydrogen/air mix ratio. The detection limits for a flame ionization detector are about 5 nanograms (ng) per second for light hydrocarbon gases, increasing to about 10 ng per second for higher organic liquids and gases, and the response is linear over approximately seven orders of magnitude.
In the prior art, the accepted techniques for determining the quantity of organic chemicals conveyed by gases have been either by using a gas chromatograph or by gravitmetric means where a weighing process is used. Such techniques usually involve determining not only the quantity of organic chemicals present in a gas, but also identifying the type of organic chemical.
It would be an advantage to have a system for detecting and measuring the quantity of known organic chemicals conveyed by gases, utilizing the principles of the flame ionization detector, wherein all of the variables affecting the detection and measurement process can be carefully controlled, including the temperatures of the various gases required in the process.
It is a principal object and advantage of the present invention to provide an apparatus for measuring the transmission rate of certain volatile organic chemicals through a barrier material. Examples of such volatile organic chemicals may be acetic acid, ethanol, methanol, pentane, propane, and xylene.
It is a further object and advantage of the present invention to provide an apparatus for testing the permeability of membranes relative to various volatile organic chemicals.
It is a further object and advantage of the present invention to provide a standardized measuring system whereby membrane permeability to volatile organic chemicals may be tested and classified.
The foregoing and other objects and advantages will become apparent from the following specification and claims, and with reference to the drawings.